Continuous evaporator

ABSTRACT

A continuous evaporator for a horizontally constructed waste heat steam generator is provided. The continuous evaporator includes a first evaporator heating surface having a plurality of essentially vertically arranged first steam generator tubes through which a flow medium can flow from bottom to top, and a second evaporator heating surface which is mounted downstream of the first evaporator heating surface on the flow medium side. The second evaporator heating surface includes a plurality of additional essentially vertically arranged second steam generator tubes through which a flow medium can flow from bottom to top. The first steam generator tubes are designed in such a manner that the average mass flow density which may be controlled in the full load operation does not fall below the predetermined minimum mass flow density in the first steam generator tubes.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the US National Stage of International Application No. PCT/EP2010/051425, filed Feb. 5, 2010 and claims the benefit thereof. The International Application claims the benefits of German application No. 10 2009 012 322.9 DE filed Mar. 9, 2010. All of the applications are incorporated by reference herein in their entirety.

FIELD OF INVENTION

The invention relates to a once-through evaporator for a horizontally constructed waste heat steam generator with a first evaporator heating surface which incorporates a number of first steam generation tubes, the arrangement of which is essentially vertical and through which the flow is from the bottom to the top, and another second evaporator heating surface, which on the flow substance side is connected downstream from the first evaporator heating surface, which incorporates a further number of second steam generation tubes the arrangement of which is essentially vertical and through which the flow is from the bottom to the top.

BACKGROUND OF INVENTION

In the case of a combined cycle gas turbine plant, the heat contained in the expanded working substance or heating gas from the gas turbine is utilized for the generation of steam for the steam turbine. The heat transfer is effected in a waste heat steam generator connected downstream from the gas turbine, in which it is usual to arrange a number of heating surfaces for the purpose of preheating water, for steam generation and for superheating steam. The heating surfaces are connected into the water-steam circuit of the steam turbine. The water-steam circuit usually incorporates several, e.g. three, pressure stages, where each of the pressure stages can have an evaporator heating surface.

For the steam generator connected downstream on the heating gas side from the gas turbine as a waste heat steam generator, several alternative design concepts can be considered, namely a design as a once-through steam generator, or a design as a recirculatory steam generator. In the case of a once-through steam generator the heating up of steam generation tubes, which are provided as evaporation tubes, results in the flow substance being evaporated in a single pass through the steam generation tubes. In contrast to this, in the case of a natural or forced circulation steam generator, the water which is fed around the circulation is only partially evaporated during its passage through the evaporator tubes. After the steam which has been generated has been separated off, the water which has not yet been evaporated is then fed once more to the same evaporator tubes for further evaporation.

Unlike a natural or forced circulation steam generator, a once-through steam generator is not subject to any pressure limitations. A high live steam pressure favors a high thermal efficiency, and hence low CO₂ emissions from a fossil-fuel fired power station. In addition, by comparison with a recirculatory steam generator, a once-through steam generator has a simple construction and can thus be manufactured at particularly low cost. The use of a steam generator, designed in accordance with the once-through principle, as the waste heat steam generator for a combined cycle gas turbine plant is therefore particularly favorable for the achievement of a high overall efficiency for the combined cycle gas turbine plant together with simple construction.

A once-through steam generator which is designed as a waste heat steam generator can basically be engineered in one of two alternative forms of construction, namely as a vertical construction or as a horizontal construction. A once-through steam generator with a horizontal construction is then designed so that the heating substance or heating gas, for example the exhaust gas from the gas turbine, flows through it in an approximately horizontal direction, whereas a once-through steam generator with a vertical construction is designed so that the heating substance flows through it in an approximately vertical direction.

Unlike a once-through steam generator with a vertical construction, a once-through steam generator with a horizontal construction can be manufactured with particularly simple facilities, and with particularly low manufacturing and assembly costs. However, in the case of a once-through steam generator with a horizontal construction, the steam generation tubes of an evaporator heating surface are exposed, depending on their positioning, to greatly differing heating. It is thereby possible that an unstable flow arises, in particular in the steam generation tubes which are upstream on the flow substance side, and this can endanger the operational reliability of the waste heat steam generator. Hence, for the purpose of dynamic stabilization there have previously been proposals, for example, for restrictors at the entry to the steam generation tubes, an enlargement of the pipe diameter from the entry towards the exit, or the use of pressure equalization lines and collectors. However, in the case of a waste heat steam generator with a horizontal construction these measures can either be ineffective or technically impossible to implement.

SUMMARY OF INVENTION

The object underlying the invention is thus to specify a waste heat steam generator of the type identified above which has a particularly high operational reliability together with a particularly simple construction.

This object is achieved in accordance with the invention in that the first steam generation tubes are designed in such a way that the mean mass flow density which is established in the first steam generation tubes when operating at full load does not fall below a prescribed minimum mass flow density.

The invention then starts from the consideration that it would be possible to achieve a particularly high operational reliability by a dynamic stabilization of the flow in the first steam generation tubes. In particular, a pulsating, oscillatory type of flow is to be avoided. Here, it has been recognized that a flow of this type arises in particular in those first steam generation tubes which are located at the heating gas side exit from the first evaporator heating surface, and which experience comparatively limited heating. These tubes contain a flow substance with a comparatively high proportion of water. Because of the greater proportionate weight of the flow substance in these tubes, the through-flow in these tubes is reduced, partly to the point of stagnation. For the purpose of avoiding this effect, it would be possible to provide chokes or pressure equalization lines, but these would mean a comparatively more expensive construction. Thus, in order to avoid stagnation of the flow and at the same time permit a particularly simple construction of the waste heat steam generator, the parameters of the steam generation tubes in the first evaporator heating surface should be directly modified. This can be achieved by designing the first steam generation tubes in such a way that the mean mass flow density through the first steam generation tubes which is established when operating at full load does not fall below a prescribed minimum mass flow density.

It is advantageous in this case if the value of the prescribed minimum mass flow density is 100 kg/m²s. That is, a design of the steam generation tubes to achieve such a choice of mass flow density leads to a particularly good dynamic stabilization of the flow in the first steam generation tubes, and hence to particularly reliable operation of the steam generator.

It has been recognized that stagnation of the flow in the tubes is caused by a comparatively large geodetic pressure loss in the steam generation tubes. In order to stabilize the mass flow density, the geodetic pressure loss should therefore be reduced as a proportion of the overall pressure loss. This can be achieved in that the internal diameter of the first steam generation tubes is advantageously chosen in such a way that the mean mass flow density which is established in the first steam generation tubes when operating at full load does not fall below the prescribed minimum mass flow density, by which means the overall pressure loss is increased by raising the frictional pressure loss.

It is advantageous if the internal diameter of the first steam generation tubes is then between 15 and 35 mm. That is, a choice of internal diameter in this range determines the volume of the first steam generation tubes to be such that the geodetic pressure loss in the steam generation tubes is so low that the mass flow density does not fall below prescribed minimum, i.e. it is no longer possible for stagnation or pulsation of the flow to occur. By this means, particularly reliable operation of the steam generator is ensured.

In an advantageous embodiment, a number of first steam generation tubes are connected one after another on the heating gas side as rows of tubes. This makes it possible to use as an evaporator heating surface a larger number of steam generation tubes connected in parallel, which means a better heat input from the enlarged surface. However, in this case the steam generation tubes which are arranged one after another in the direction of flow of the heating gas are differently heated. Particularly in the steam generation tubes on the heating gas exit side, the flow substance is comparatively weakly heated. By the design described for the steam generation tubes it is however also possible to avoid stagnation of the flow in these steam generation tubes. By this dynamic stabilization, particularly reliable operation is achieved for the waste heat steam generator together with a simple construction.

In an advantageous embodiment, the first evaporator heating surface is connected downstream on the heating gas side from the second evaporator heating surface. This offers the advantage that the second evaporator heating surface, which is connected downstream on the flow substance side and is thus designed to further heat up a flow substance which has already been evaporated, also lies in a comparatively strongly heated region of the heating gas duct.

A once-through evaporator of this type can expediently be used in a waste heat steam generator, and the waste heat steam generator used in a combined cycle gas turbine plant. In this case it is advantageous to connect the steam generator downstream on the heating gas side from a gas turbine. With this connection, a supplementary heat source can expediently be arranged behind the gas turbine, to raise the heating gas temperature.

The advantages achieved by the invention consist, in particular, in the fact that designing the first steam generation tubes in such a way that the mean mass flow density established in the first steam generation tubes when operating at full load does not fall below a prescribed minimum mass flow density achieves a dynamic stabilization of the flow, and thus particularly reliable operation of the waste heat steam generator. By an appropriate design of the steam generation tubes this effect is achieved even without further expensive technical measures, and thus permits at the same time a particularly simple, cost-saving construction for the waste heat steam generator or a combined cycle gas turbine power station, as applicable.

BRIEF DESCRIPTION OF THE DRAWINGS

An exemplary embodiment of the invention is explained in more detail by reference to a drawing. The FIGURE in the drawing shows a simplified representation of a longitudinal section through a steam generator with a horizontal construction.

DETAILED DESCRIPTION OF INVENTION

The once-through steam generator 1 for the waste heat steam generator 2 shown in the FIGURE is connected downstream from a gas turbine, not shown here in more detail, on its exhaust gas side. The waste heat steam generator 2 has a surrounding wall which forms a heating gas duct 5 through which the exhaust gas from the gas turbine can flow in an approximately horizontal direction as heating gas, as indicated by the arrows 4. Arranged in the heating gas duct 5 is a number of evaporator heating surfaces 8, 10, designed according to a once-through principle. In the exemplary embodiment shown in FIG. 1, each of two evaporator heating surfaces 8, 10 is shown, but a larger number of evaporator heating surfaces could also be provided.

Each of the evaporator heating surfaces 8, 10 shown in the FIGURE incorporates a number of rows of tubes, 11 and 12 respectively, each in the nature of a nest of tubes, arranged behind each other in the direction of the heating gas. Each row of tubes 11, 12 incorporates in turn a number of steam generation tubes, 13 and 14 respectively, in each case arranged beside each other in the direction of the heating gas, of which in each case only one can be seen for each row of tubes 11, 12. The first steam generation tubes 13 of the first evaporator heating surface 8, which are arranged approximately vertically and connected in parallel so that a flow substance W can flow through them, are here connected on their output sides to an outlet collector 15 which is common to them. The second steam generation tubes 14 of the second evaporator heating surface 10, which are also arranged approximately vertically and connected in parallel so that a flow substance W can flow through them, are also connected on their output sides to an outlet collector 16 which is common to them. For flow purposes, the steam generation tubes 14 of the second evaporator heating surface 10 are connected downstream from the steam generation tubes 13 of the first evaporator heating surface 8, via a downpipe 17.

The evaporation system formed by the evaporator heating surfaces 8, 10 can have admitted to it the flow substance W which, in a single pass through the evaporation system, is evaporated and after it emerges from the second evaporator heating surface 10 is fed away as steam D. The evaporation system formed by the evaporator heating surfaces 8, 10 is connected into a steam turbine's water-steam circuit, which is not shown in more detail. In addition to the evaporation system which incorporates the evaporator heating surfaces 8, 10, the water-steam circuit of the steam turbine has connected into it a number of other heating surfaces 20, indicated schematically in the FIGURE. The heating surfaces 20 could be, for example, superheaters, medium-pressure evaporators, low-pressure evaporators and/or preheaters.

The first steam generation tubes 13 are now designed in such a way that the mass flow density does not fall below a minimum prescribed for full load operation as 100 kg/m²s. Here, their internal diameter is between 15 mm and 35 mm. By this means, stagnation of the flow in the first steam generation tubes 13 is avoided. A standing column of water with the formation of steam bubbles, and a resulting oscillatory type of pulsating through-flow, is prevented. By this means, the mechanical loading on the waste heat steam generator 2 is reduced, and particularly reliable operation is guaranteed at the same time as a simple construction. 

1.-8. (canceled)
 9. A continuous evaporator for a horizontally constructed waste heat steam generator, comprising: a first evaporator heating surface which incorporates a plurality of first steam generation tubes, a first arrangement of which is essentially vertical and through which a flow is from the bottom to the top; and a second evaporator heating surface, which on a flow substance side is connected downstream from the first evaporator heating surface and incorporates a plurality of second steam generation tubes, a second arrangement of which is essentially vertical and through which the flow is from the bottom to the top, wherein the plurality of first steam generation tubes are designed in such a way that a mean mass flow density which is established in the plurality of first steam generation tubes when operating at full load does not fall below a prescribed minimum mass flow density.
 10. The continuous evaporator as claimed in claim 9, wherein a value of the prescribed minimum mass flow density is 100 kg/m²s.
 11. The continuous evaporator as claimed in claim 9, wherein an internal diameter of the plurality of first steam generation tubes is chosen such that the mean mass flow density which is established in the plurality of first steam generation tubes when operating at full load does not fall below the prescribed minimum mass flow density.
 12. The continuous evaporator as claimed in claim 11, wherein the value of the internal diameter of the plurality of first steam generation tubes is between 15 mm and 35 mm.
 13. The continuous evaporator as claimed in claim 9, wherein the plurality of first steam generation tubes are connected one after another on a heating gas side as rows of tubes.
 14. The continuous evaporator as claimed in claim 9, wherein the first evaporator heating surface is connected downstream on the heating gas side from the second evaporator heating surface.
 15. A waste heat steam generator, comprising: a continuous evaporator as claimed in claim
 9. 16. The waste heat steam generator as claimed in claim 15, wherein upstream from the waste heat steam generator on a heating gas side is connected a gas turbine.
 17. The waste heat steam generator as claimed in claim 15, wherein a value of the prescribed minimum mass flow density is 100 kg/m²s.
 18. The waste heat steam generator as claimed in claim 15, wherein an internal diameter of the plurality of first steam generation tubes is chosen such that the mean mass flow density which is established in the plurality of first steam generation tubes when operating at full load does not fall below the prescribed minimum mass flow density.
 19. The waste heat steam generator as claimed in claim 18, wherein the internal diameter of the plurality of first steam generation tubes is between 15 mm and 35 mm.
 20. The waste heat steam generator as claimed in claim 15, wherein the plurality of first steam generation tubes are connected one after another on a heating gas side as rows of tubes.
 21. The waste heat steam generator as claimed in claim 15, wherein the first evaporator heating surface is connected downstream on the heating gas side from the second evaporator heating surface. 